Fuel cells are electrochemical devices in which electricity is produced by a direct reaction of a fuel and an oxidizer. Fuel cells, such as those based on polymer electrolyte membranes, also called proton exchange membranes (PEMs), are highly efficient, nonpolluting and silent in operation. Hence, PEMFCs are becoming an increasingly important technology, with both stationary applications such as block power stations for on-site power generation and mobile applications such as automotive, industrial, transportation, and mobile communications.
Problems impeding the development of fuel cell technology mainly involve electrocatalysts and electrolyte membranes used within the fuel cells. In the operation of a fuel cell, electrochemical oxidation and/or reduction reactions take place at the surface of the electrodes. Poisoning of catalysts used within fuel cells, such as platinoid catalysts, platinum-based catalysts and the like, can occur in fuel cells at usual operating temperatures, which are typically below around 80° C., due to properties of the membrane material. Accordingly, there is a motivation to increase the operating temperature of fuel cells to greater than about 100° C. in order not only to enhance the reaction kinetics, since reaction rates increase with temperature, but also to enhance electrode tolerance to species such as carbon monoxide, which is a byproduct of fuel processing, which can poison or otherwise degrade the electrode surface.
It is the presence of solvents, such as water, in the electrolyte membranes of most current fuel cells that limits the fuel cell's performance. In conventional fuel cells, the membranes separating the electrodes rely heavily on water as a proton charge carrier. This reliance limits the operating temperature range of such fuel cells to less than 100° C. at atmospheric pressure otherwise the water evaporates, killing the conductivity. Currently, the state of the art PEM material used in fuel cells is Nafion (DuPont, United States). PEMFCs utilizing Nafion to form the PEM are limited to an operating temperature below or near 80° C. in order to maintain proper hydration of the PEM. Proper hydration of a Nafion PEM is critical for good proton conductivity and overall fuel cell performance. Low temperatures also are required to maintain the material integrity of Nafion, which is lost at temperatures near 100° C.
Most existing PEMs also require high purity hydrogen (H2) as a fuel in order to function correctly. The ability to operate at temperatures above approximately 140° C. would allow the use of a broader range of more widely available and lower-cost materials, including impure hydrogen, and hydrocarbons such as methanol or the like, and would make the PEMs less susceptible to contamination. Operation at temperatures greater than 100° C. also would permit a simplified system with all-gas fluidics, since materials such as water exist as a vapor at these high temperatures. Finally, low temperature operation of existing PEM-based fuel cells does not allow waste heat to be utilized. The PEM cannot be used in conjunction with a reformer that can capture and utilize the heat to increase fuel cell efficiency further.
In order to overcome the limitations associated with conventional fuel cell membranes, two classes of PEMs have been proposed: phosphoric acid fuel cells and solid acid fuel cells. Problems are associated with both of these types of membranes. Although phosphoric acid-loaded polybenzimadole PEM electrolytes are able to operate at higher temperatures, the fuel cell stability is limited due to electrolyte adsorption on platinum (Pt) catalysts resulting in low voltage generation. The solid acid membranes have similar hydrolysis and stability limitations in the presence of liquid water, resulting in reduced efficiency.
General considerations for electrolytes used in fuel cells are (1) electrolyte must be a conductor for protons, preferably with a conductivity for hydrogen ions (H+) greater than approximately 10−2 Siemen/cm; (2) electrolyte must be an insulator for electrons, with a conductivity for electrons (e31) less than approximately 10−9 Siemen/cm; (3) electrolyte must operate at temperatures in the range approximately between −50 and 230° C.; (4) electrolyte must be dimensionally stable at temperatures in the range approximately between −50 and 230° C.; (5) electrolyte must be stable to aqueous acid and alkaline media; (6) electrolyte must be stable under reducing potentials in the presence of a catalyst such as Pt at temperatures up to about 230° C., and in the presence of H2 or a hydrocarbon on the catalyst; and (7) electrolyte must be stable under oxidizing environments in the presence of a catalyst such as Pt at temperatures up to about 230° C., and in the presence of oxygen (O2), which can be from the air, on the catalyst.
Additional requirements exist for membranes used in fuel cells (1) membrane must have a low permeability to H2 and O2 gas; (2) membrane should not allow electroosmosis (i.e., transfer of water with a proton); (3) membrane must be dimensionally stable with a change of hydration state; (4) membrane must have a low thermal expansion coefficient; (5) membrane must have good cohesion with or adhesion to electrodes used in the fuel cell; and (6) membrane must be free of defects such as pin-holes to ensure no or very low reactant cross-over and no electrode shorting.
In view of the problems with prior art membranes, there is a need for improved proton-transporting membranes capable of sustaining high and stable conductivity at temperatures greater than 100° C. without requiring additional humidification systems or hydrating water.